The present invention relates generally to a method for measuring the subscriber area of an Integrated Services Digital Network (ISDN) system, and more particularly to such a method for measuring how a component of a subscriber area of a standard ISDN system reacts to selected data sequences by disconnecting the subscriber data terminal and/or the net termination from the data bus and connecting a test pattern generator and a measuring device to the data bus.
An ISDN system can be subdivided into a so-called subscriber area and the ISDN user area. The boundary between these two areas is formed by a net termination. A data bus line leads from this net termination through the subscriber's space. The net termination or the data bus is provided with a standardized interface, the so-called S.sub.o -interface, to which different types of terminal equipment can be connected. This S.sub.o -interface is a four-wire interface for bidirectional data exchange between the net termination and one or several of the subscriber's data terminals. A so-called AMI-NRZ code is used as a transmission code. This is a quasi ternary code, in which the "0" bits are represented with +0.75 V or -0.75 V and the "1" bits are represented as zero volts; consecutive "0" bit pulses have fundamentally alternating polarity. The data to be transmitted, as well as synchronization and control data, are compressed into transmission frames that are 250 .mu.s long. The beginning of such a transmission frame is characterized by a code violation. The code violation occurs when two consecutive "0" bit pulses have the same polarity. This violation is positively recognized by the data terminal, so that the data terminal is easily and quickly synchronized with the net termination.
Before a data terminal can receive or transmit data, it is necessary to set up an appropriate connection by means of a defined procedure. Thus, besides the already mentioned synchronization of the transmission frames, the transmission channels must be made transparent, in other words they must be prepared for transferring data. This procedure, also designated as activation, consists of the two-way transmission and reception of specific, defined data sequences--in the following also denoted as infos--between the data terminal and the net termination. To this end, these components are provided with special control modules, S.sub.o bus controllers (SBC) and ISDN communication controllers (ICC). The SBC modules control the activation and also the deactivation procedures respectively. This control system is monitored by the internal rate setting of timers; this means that the corresponding infos are always only output for a specific period of time; within this time, a specific info must be transmitted by the corresponding component as a response to a received info. The info to be transmitted as a response from the corresponding component is specifically meant for the info that the component had just received before, so that each procedure is characterized by a defined hierarchy of infos. If the connection set-up (activation), as mentioned above is supposed to start from a data terminal, then this data terminal initially transmits a first data sequence, designated as info 1, to the net termination. In the normal case, this net termination reacts to the reception of the data sequence, info 1, by transmitting a data sequence designated as info 2; this response must take place within a specific time and the transmission of info 2 ensues only for a limited time. During this time, on the other hand, the terminal must react to the reception of info 2 by transmitting a data sequence, info 3, whose reception, in turn, is acknowledged by the net termination by the transmission of a data sequence, info 4. If the info transmitted as a response by the corresponding component does not correspond to the info to be expected in accordance with the hierarchy (for example transmission of info 4 and response to info 1), the expected info is not received within a defined time, or if the procedure is not begun from the subscriber data terminal with info 1, if then the procedure is discontinued and the S.sub.o data bus once again finds itself in its initial state, that is in an inactive state.
This performance characteristic that is necessitated by the SBC or ICC modules (for example according to the West German post office guideline 1 TR3, that includes instructions for measuring on the S.sub.o interface) makes it more difficult to measure on the communication paths and on the S.sub.o interfaces. An info is available for a limited time only--namely for a defined, maximum transmit duration or until the info expected as a response is received, so that only a limited time is available as well to measure the reaction of the corresponding component. Furthermore, in the case of the measurements, one is bound to the hierarchical sequence of the infos, as they occur in a specific procedure. Moreover, these modules only provide specific, defined infos that are used in the operational procedures of a system.
These difficulties also result when a Siemens Protocol Tester K 1195 is used in place of a net termination or a subscriber data terminal, as described in the Siemens Telcom Report 11 (1988) March-April, No. 2, pp 61 to 64, particularly from page 62, right column, last two paragraphs to page 64, first three paragraphs. In the case of the measurement described there, one does indeed measure at the S.sub.o interface, after the subscriber data terminal or the net termination has been disconnected from the data bus found in this area, and the Protocol Tester K1195 has been connected, and the area has also been activated by the infos. However, in doing this, only protocol measuring tasks are solved; there is no measurement in the physical plane.